Effect Of Sr,Ca On Microstructure And Properties Of Magnesium Alloys

Because of its low cost and excellent heat resistance, heat resistant Mg-Zn alloy hasaroused widespread attention. A small amount of Ca, Sr and other rare earth elements arealways added to ZA alloys （ZA104, ZA144, ZA106, ZA85, ZA84） to improve the structureand property. In this paper, design the two series magnesium alloy （Mg-Zn-Al-Si-Sr andMg-Zn-Al-Si-Ca alloys） were designed by adding Si, Sr and Ca elements into theMg-8Zn-5Al alloy. The microstructure and phase composition of the alloys were analyzedby optical microscopy （OM）, scanning electron microscopy （SEM）, X-ray diffraction（XRD）. The mechanical properties of alloys were tested by universal electronic materialtesting machine and hardness tester and the fracture surface was analyzed by scanningelectron microscopy （SEM）. Solution and aging were treated by pit furnace. The importantresults are summarized as following:The as-cast ZAS851alloy is mainly composed of divorced eutectic-Mg₃₂（Al, Zn）₄₉phase, regular eutectic τ-Al₂Mg₅Zn₂phase and continuous bulk Mg₂Si phase. Theformation of Mg₂Si phase was restrained, and τ phases were reduced, while therod-shaped or needle Al4Sr and Mg17Sr2phase were increased with Sr addition. Irregularblocks, flakes and rules mesh τ phases were into thin strips, τ phases, and reduced,while massive, polygon massive Al2Ca, CaMgSi phase were increased with Ca addition. Srand Ca both can refine primary phase α-Mg, meanwhile Sr changes the the type,morphology and distribution of precipitated phase significantly and Ca changes the type ofprecipitation phase mainly.The as-cast Mg-8Zn-5Al-1Si-2Sr alloy has the highest tensile strength, yield strengthand elongation of185.85MPa,147.68MPa and3.6%respectively, which is the best oneamong the ZAS851-xSr（x=0,1,2,3） alloys. The mechanical properties of the alloys have been significantly improved after heat treatment. The most suitable heat treatment processfor Mg-8Zn-5Al-1Si-2Sr alloy is solution for20h at350℃and then agingfor24h at180℃.The tensile strength, yield strength, elongation and brinell hardness ofMg-8Zn-5Al-1Si-2Sr alloy are190.87MPa、152.34MPa、4.3%and53HB respectively afterheat treatment.The as-cast Mg-8Zn-5Al-1Si-2Ca alloy has the highest tensile strength, yield strengthand elongation of175.85MPa,145.88MPa and4.3%respectively, which is the best oneamong the ZAS851-xCa（x=0,1,2,3） alloys. The mechanical properties of the alloys havebeen significantly improved after heat treatment. The most suitable heat treatment processfor Mg-8Zn-5Al-1Si-2Ca alloy is solution20h at350℃and then aging24h at200℃. Thetensile strength, yield strength, elongation and brinell hardness of Mg-8Zn-5Al-1Si-2Sralloy are180.27MPa、150.34MPa、4.3%and50HB respectively after heat treatment.